1. Technical Field of the Invention
The present invention relates to electrical rotating machines, whether motors or generators, and more precisely to the stators thereof.
2. The Related Art
In a known construction, the stator of machines of the foregoing type has a magnetic circuit and windings of electrically conductive wires which in general are made from insulated copper wire, often round in cross section. The magnetic circuit, for its part, is laminated; i.e., it is formed by a stack of magnetic metal sheets. Each metal sheet is cut to shape such that slots delimited by teeth are made, the slots being the seatings for the electrically conductive wires. This principle of arranging the stator is widely applied in synchronous or asynchronous machines.
There are applications for which it is desirable to simultaneously obtain both high power levels and highly compact constructions of the motor. To give just one concrete example, when the intention is to install electric traction motors in the wheels of automotive vehicles, it is desirable to be able to develop power levels of at least 10 kW per motor, and even at least 25 or 30 kW per motor for the majority of the time, for a weight as low as possible in order not to make the unsprung masses too heavy. It is also desirable for the bulk to be very small, not going beyond or going beyond by as little as possible, the internal volume of the wheel, so that the motor does not interfere with the elements of the vehicle in the event of flexing of the suspension and in the event of other types of movement of the wheel with respect to the vehicle body.
These requirements (high power level and low weight and bulk) make it very problematic to install electric traction motors in the wheels of private vehicles without radically improving the ratio of weight to power of the electrical machines currently available on the market.
Moreover, the heating caused by losses of the motors must be contained within certain limits, or else irreversible degradation occurs, in particular with respect to the insulation of the electrical conductors. The heat produced in the stator conductors must thus be dissipated as effectively as possible.
In the most demanding applications, it is already well known to cool electrical rotating machines by means of a liquid. In this case, forced circulation of the liquid is provided within the electrical rotating machine itself, principally the stator, in which the electrical windings are located, so that the heat is guided out to a heat exchanger.
In order properly to hold the electrical conductors mechanically in their slots, to reinforce the electrical insulation and indeed to contribute to good heat exchange by conduction, it is already known to impregnate the electrical conductors in resin, which fills the various slots and covers the winding heads on either side of the stator.
Unfortunately, although impregnation of the conductors with resin proves indispensable in high-performance motors, the resins used to impregnate the conductors in the slots are materials which are relatively poor heat conductors. It is also known to use resins containing fillers, which are better conductors of heat, to impregnate the conductors of the winding heads. Unfortunately, the resins used in the winding heads are not suitable for impregnating the conductors in the slots. The fillers cannot penetrate into the small spaces remaining in the slots, and this is all the more true in high-performance motors because a high level of filling the slots with copper is sought.
Furthermore, dissipation of the heat produced in the slots is problematic. In fact, there are currently faults in the impregnation; in other words, bubbles (of air or degassing products) remain trapped inside the mass of the conductors and resin. The consequence is that the heat exchange taking place where these faults in the impregnation exist is much less effective, since it cannot take place by conduction. The result is a local rise in temperature, which can have an adverse effect on the proper behaviour of the electrical insulating materials used and, as a consequence, cause thermal breakdown phenomena in the electrical rotating machines.